Allow to configure the Icinga Web 2 Iframe URL
This is a little brainfuck, as we need to create an instance of the icinga2 class for reading the configuration details in `config.ru`. We'll expose the setting inside a helper function, which is called inside the HTML erb template for the dashboard. Note: This is done every time the user presses refresh. Job execution won't trigger this. The previous attempt with updating this on send_event caused flickering.